1. Field of The Invention
The present invention relates to an image reading apparatus of sheet-through type wherein a document is fed through a read position to read a document image by a stationary reading device provided at the read position.
2. Description of Prior Art
In a type of an image reading apparatus, a document put on a platen glass is read by moving an exposure lamp and an image sensor along a subscan direction. The position of a top end of the document fed onto the platen glass by the automatic document feeder is different among automatic document feeders. In order to correct the scattering of the position of the top end, an adjustment value on the position of the top end of a document has been set for each automatic document feeder, as described in Japanese Patent laid open Publication 4-292,064/1992. Then, a timing to start reading with the image sensor is corrected in correspondence to the adjustment factor. Thus, a position shift in an image read by the sensor and reproduced on a paper can be prevented.
On the other hand, in sheet-through type image reading apparatuses described in Japanese Patent laid open Publications 1-265,666/1989 and 1-167,766/1989, a document fed by an automatic document feeder is read with an image sensor which is fixed at a read position.
A prior art sheet-through type image reading apparatus is known to start reading with the sensor when a document which is fed by a document feeder from a standby position reaches the read position. In such an apparatus, after the apparatus starts preparation operations such as warming-up of the exposure lamp, a certain time is needed until the image sensor is ready for reading an image. On the other hand, a certain feed time t1 is needed for a document to be carried from the standby position to the read position, and the carriage time depends on operation conditions and the like. For normal reading, it is needed that preparation for reading has to be completed before the reading is started. Then, in prior art image reading apparatuses, after a document is kept at the standby position for a standby time, the document is started to be fed at a timing earlier by T1min than the time when the preparation for reading is completed, wherein T1min is a minimum of T1 in various operation conditions of the document feeder. Thus, it is prevented that the document reaches the read position before the preparation operation for reading is completed.
As described above, in the prior art sheet-through type image reading apparatus, a document is started to be fed always at the timing earlier by the minimum time T1min than the time when the preparation for reading is completed, irrespective of operation conditions. Then, in an operation condition where the actual document feed time T1a is longer than the minimum T1min, a time loss occurs after the preparation condition for image reading is completed, because a certain time is waited for the document to reach to the read position. That is, if document is fed in an operation condition that t1=T1a ( greater than T1min), the document reaches the read position when a time of (T1axe2x88x92T1min) elapses further after the preparation operation completes. Thus, a time loss of (T1axe2x88x92T1min) occurs.
An object of the present invention is to provide an imaging reading apparatus which can start reading at an appropriate timing to improve read efficiency.
In one aspect of the invention, an image reading apparatus has a reading device for reading a document fed by a document feeder. The reading device is correlated to the document feeder so that the reading is started at an earlier timing which takes scattering of the magnifying power or feed length among apparatus into account as described below. Operation conditions for reading are set with a setting device. The document feeder starts to feed a document from a start position and passes a read position at a speed corresponding to the operation conditions. The image reading device reads the document fed by the document feeder at the read position after preparation for reading based on the operation conditions is completed. A first time is calculated which is earlier by a period needed for the document feeder to feed a document from the start position to the read position than a second time when the preparation for reading completes, and the first time is set for the document feeder to start to feed a document.
In a second aspect of the invention, an image reading apparatus also has an image reading device for reading a document fed by a document feeder. On the other hand, a correction value on a feed length from the start position to the read position is set, wherein the feed length scatters for each image reading apparatus. Then, a period is calculated needed for the document feeder to feed a document from the start position to the read position based on a design feed length and the correction value, and a first time is calculated earlier by the period than a second time when the preparation for reading completes. Then, the first time is set for the document feeder to start to feed a document. The document feeder starts to feed a document from a start position to pass the read position at a predetermined speed. The image reading device reads the document at a predetermined read position after preparation for reading based on the operation conditions is completed.
An advantage of the present invention is that a document can be read efficiently.